The banded iron formation (BIF) of the Neoarchean (2.76-2.74 Ga) Kostomuksha Formation of the Kostomuksha greenstone belt of the Karelian Craton was shown to contain the following accessory minerals: zircon, apatite, monazite and barite. Zircons from BIF have 2.9-2.8 Ga magmatic cores indicating the presence of Mesoarchean rocks in source areas. Metamorphic zircons were dated at 2.73, 2.64 and 1.86 Ga. They mark the polymetamorphic evolution.
Early Paleoproterozoic stromatolite assemblages were first described in Karelia, Russia and Finland and they were later adopted for stromatolite biostratigraphy in China, India, and Australia. However, their biostratigraphic potential is not widely accepted due to their strong provinciality and apparently different time ranges for taxa on global scale. Stratigraphic potential of stromatolites was further hampered by recent controversy over their biogenic origin, in particular of microdigitate stromatolites. Here we compare microdigitate stromatolites from carbonate successions deposited during the ca. 2.2-2.1 Ga carbon isotope excursion to test whether they contain similar taxa. We assign some Paleoproterozoic microdigitate stromatolites from Finland and USA that were earlier described as different genera to genus Djulmekella originally defined in Karelia, Russia, in the eastern part of the Fennoscandian Shield. Our study suggests that microdigitate stromatolites with similar morphologies are abundant in the 2.2-2.1 Ga carbonate successions and emphasize their potential for interbasinal correlation.
ABSTRACTZircon is not a common accessory mineral of banded iron formation (BIF), but it occurs in all BIF samples from the Kostomuksha Greenstone Belt (KGB) in the Karelian Craton of the Fennoscandian Shield. The Kostomuksha BIF is of Algoma type and is part of three variably old rock associations: BIF-1 is part of a sedimentary sequence in Mesoarchean (2.87–2.84 Ga) basaltic komatiites with dacites, BIF-2 is located in Meso-Neoarchean (2.8–2.79 Ga) felsic volcanics, and BIF-3 is part of a Neoarchean (2.76–2.74 Ga) greywacke unit with felsic volcanics. Analysis of zircon has revealed grains with cores and several generations of rims. The U-Th-Pb systems of zircon indicate that its isotopic age varies considerably from 2.98 to 1.89 Ga, forming up to 4 age clusters in each sample. Most samples contain single inherited grains. The bulk of zircon in BIF is younger than the host rocks and is interpreted as metamorphic. Most of metamorphic zircons display an oscillatory zoning, and their Th/U ratio varies from 0.01 to 4.7, i.e. their origin is hard to interpret reliably, based on the above characteristics. Paleoproterozoic (1.89–1.85 Ga) zircons from BIF-1 and 3, occurring as rims and individual grains, are related to local tectono-thermal processes simultaneous with the formation of the Svecofennian orogen and were described for the central Karelian Craton for the first time. Thus, both inherited and metamorphic zircons have been revealed in Meso-Neoarchean BIF metamorphosed repeatedly in the Neoarchean-Paleoproterozoic under up to amphibolite-facies conditions. All metamorphic generations of zircons from BIF are correlated with the tectono-thermal events in the Karelian Craton, including the effect of the Svecofennian orogeny.KEYWORDS: Banded iron formationzirconsisotopic ageArcheanPaleoproterozoicKostomuksha greenstone beltKarelian craton AcknowledgmentsThe authors are grateful to K.G. Erofeyeva, V.S. Sheshukov and A.S. Dubenskiy of GIN RAS (Moscow) for zircon analyses and discussion of the data. The authors also wish to thank Dr. Robert J. Stern, Dr. Chris Yakymchuk and two anonymous reviewers for comments that have helped improve the manuscript, as well as G. Sokolov who translated the manuscript.Prof. Anthony Robert Prave from the University of Saint Andrews is gratefully acknowledged for editing the authors’ English translation.Disclosure statementNo potential conflict of interest was reported by the author(s).Supplementary materialSupplemental data for this article can be accessed online at https://doi.org/10.1080/00206814.2023.2248501Correction StatementThis article has been corrected with minor changes. These changes do not impact the academic content of the article.Additional informationFundingThis work was supported by the Russian Science Foundation under Grant number 22-17-00026, https://rscf.ru/project/22-17-00026/.
Abstract The Palaeoproterozoic Lomagundi-Jatuli Event (LJE) is the largest positive carbonate C-isotope ( δ13Ccarb) excursion in Earth history. Conventional thinking is that it represents a perturbation of the global C cycle between c. 2.3–2.1 Ga linked directly to the postulated Great Oxidation Event. Here we show that the LJE worldwide is, in fact, entirely facies (i.e. palaeoenvironment) dependent. During the LJE, the C-isotope composition of open and deeper marine settings remained undisturbed, with a mean d13Ccarb value of 1.5‰, whereas nearshore marine and coastal-evaporitic settings have means of 6.2‰ and 8.1‰, respectively. This finding refutes conventional thinking and obliges complete re-evaluation of concepts using the LJE as a means of assessing oxygenation of the atmosphere.
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